biochemistry - What Biology topics are linked to the Nitrogen Cycle?

The nitrogen cycle is a highly biological process. Nitrogen gas is ~75% of the earth's atmosphere and N₂ is the most chemically stable form of Nitrogen for conditions in the Earth's crust.

Nitrogen is one of the four most essential and universal elements to terrestrial life (Carbon Hydrogen and Oxygen being the others). Nitrogen fixation, the name given to the various processes which make chemically reactive forms of Nitrogen available to living things. Ammonia (NH₃ / NH₄⁺) and Nitrites / Nitrates (NO₂^-, NO₃^-) are chemically unstable compared to N₂ and the conventional biosynthetic processes in the cell can use any of these forms to make proteins, nucleic acids and other biologically active molecules necessary for life.

Ammonia and similar compounds like Urea come from the biological byproducts of living cells - waste. These are readily recycled in the soil by plants and microorganisms and I imagine some animals and insects. Nitrogen is a limiting resource for living things and most biomes are pretty efficient at recycling reduced nitrogen from primary (NH₃) secondary (NHR₂), tertiary (NR₃) and quaternary amines (NR₄).

But the biosphere has a great demand for bio-useful nitrogen and in most places on earth in deep water and soil microorganisms can break down N₂ directly into nitrates/nitrites. This happens deep underwater or underground because the enzymes which perform this activity are delicate and ruined by O₂. This process is entirely performed by anaerobic microorganisms. In recent decades, the ecological importance of subterranean microbial communities has become better appreciated for the complex structure and ecological roles it plays.

An interesting side note to this is that up to the early 20th century, the growth of life and human civilization was really limited by the intense labor required to grow food. While hydrogen oxygen and carbon are essentially free - obtained from water, atmospheric oxygen and CO₂ by plants, Nitrogen was the limiting factor in many cases to growing more crops or animals for food.

The Haber process, which converts nitrogen and hydrogen gas under high temperature and pressure into ammonia over a catalyst was such an important discovery - it allowed the industrial preparation of effective fertilizers and was instrumental in creation of the green revolution.